The present invention generally relates generally to the field of disc drives, and more particularly to an apparatus and method for providing a reliable, ferrofluidic seal between a hub and a shaft of a spindle motor in a disc drive.
Disc drives, including magnetic disc drives, optical disc drives and magneto-optical disc drives, are widely used for storing information. A typical disc drive has one or more discs for storing information in a plurality of concentric circular tracks. This information is written to and read from the discs using read/write heads mounted on actuator arms which are moved from track to track across surfaces of the discs by an actuator mechanism. The discs are mounted on a spindle which is turned by a spindle motor to pass the surfaces of the discs under the read/write heads. The spindle motor generally includes a shaft fixed to a baseplate and a hub, to which the spindle is attached, having a sleeve into which the shaft is inserted. Permanent magnets attached to the hub interact with a stator winding on the baseplate to rotate the hub relative to the shaft. Bearings between the hub and the shaft facilitate rotation of the hub.
The spindle motor also typically includes an exclusion seal to seal interfacial spaces between the hub and shaft. This is necessary, because lubricating fluids or greases used in the bearings tend to give off aerosols or vaporous components that migrate or diffuse out of the spindle motor and into a disc chamber in which the discs are maintained. This vapor often transports other particles, such as material abraded from the bearings or other components of the spindle motor, into the disc chamber. These vapors and particles deposit on the read/write heads and the surfaces of the discs, causing damage to the discs and the read/write heads as they pass over the discs. Thus, the migration of these contaminants into the disc chamber must be prevented.
To prevent the migration of these contaminants into the disc chamber, the latest generation of spindle motors utilize a ferrofluidic seal between the shaft and the hub. Ferrofluidic seals are described in, for example, U.S. Pat. No. 5,473,484, which is incorporated herein by reference. A typical ferrofluidic seal consists of ferrofluid, an axially polarized annular magnet and two magnetically permeable annular pole pieces attached to opposing faces of the magnet. The ferrofluid is conventionally composed of a suspension of magnetically permeable particles suspended in a fluid carrier. Generally, the magnet and the pole pieces are fixed to the hub and extend close to but do not touch the shaft. Magnetic flux generated by the magnet passes through the pole pieces and the shaft, which is also magnetically permeable, to magnetically hold the ferrofluid in gaps between the pole pieces and the shaft, thereby forming a seal.
As noted above, current ferrofluidic seals use a rotating design in which the magnet and pole pieces are affixed to the hub, and a ferrofluid seal is found between the rotating pole pieces and the fixed shaft. Although this design has worked well for conventional spindle motors, the latest generation of motors rotate at high speeds, often exceeding 10,000 revolutions per minute (rpm). The centrifugal forces developed at such high speeds often exceed the ability of the ferrofluidic seal""s magnetic flux to hold the ferrofluid against the shaft due to the velocity gradient across the ferrofluid, resulting in the failure of the ferrofluidic seal.
Accordingly, there is a need for a ferrofluidic seal that is reliable at high rotational speeds.
The present invention provides a solution to these and other problems, and offers other advantages over the prior art.
The present invention relates to an apparatus for sealing the outer surface of a shaft to a face of a hub disposed about the shaft that solves the above problems.
In yet another aspect, the present invention is directed to a ferrofluidic seal for sealing a stationary shaft to a rotating hub. A formed magnet is attached to the rotating hub, bounded by upper and lower members to form a shaped cavity. A stationary ring is attached to the shaft and extends into the cavity, which is filled with a conductive ferrofluid. The cavity has a uniform, concave cross section over at least part of its surface. The end of the stationary ring distal from the shaft is shaped so that in the region containing the fluid, the ring and cavity surface are equidistant from a set of points defining the seals xe2x80x9ccharacteristic contourxe2x80x9d. The seal""s Power Index is defined as (Axc3x97R{circumflex over ( )}2)/(xcex94xc3x97V), where:
A=surface area of the characteristic contour
R=maximum radius of the contour about the axis of rotation
xcex94=average gap between the inner and outer member
V=total volume of the fluid
Power loss is minimized by constructing the seal so that the Power Index is less than 700 (more preferably in the range of about 200 to 500). This differs from prior systems by allowing very high rotation rates with acceptable power consumption.
The current invention enables seals to be operated at high rotational velocities at acceptable power levels.
The present invention is particularly useful in a spindle motor such as used in a disc drive. A spindle motor generally has a base supporting a shaft, and a hub having an inner surface disposed about an outer surface of the shaft. An embodiment of a seal according to the present invention is positioned between the shaft and the hub to seal the outer surface of the shaft to the inner surface of the hub and to electrically couple the shaft to the hub.
These and various other features as well as advantages which characterize the present invention will be apparent upon reading of the following detailed description and review of the associate drawings.